Spectroscopy and photometry of HR Lyr (Nova Lyr 1919) during one of its mysterious dimming episodes

ATel #9418; U. Munari (INAF Padova), A. Siviero, P. Ochner (Univ. Padova); S. Dallaporta (ANS Collaboration)
on 27 Aug 2016; 14:17 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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An alert posted to the BAA-VSS newsgroup on Aug 24 by J. Shears informs about a new excursion down to V~17 of old nova HR Lyr (Nova Lyr 1919). A few other such episodes were catch by Honeycutt et al (2014, AJ 147, 105) in their intensive 1991-2012 V-band monitoring of HR Lyr, usually between V=15.5 and 16.0. Apparently no multi-color photometric or spectroscopic observations obtained during one such dimming episode have been published, which contributes to the uncertainty about their causes. We have obtained BVRI photometry of HR Lyr on Aug 25.94 UT with the Asiago Schmidt 67/92cm telescope, and measured V=16.98, B-V=+0.31, V-Rc=+0.36 and V-Ic=+0.86 (+/- 0.01 on all of them) on the Landolt (2013, AJ 146, 131) system of standards at +50 dec. This confirms the low state HR Lyr is currently undergoing and founds its color slightly redder than the average B-V=+0.13 listed by Bruch (1984, A&AS 56, 441). We have obtained a 3300-8000 Ang spectrum (dispersion 2.31 Ang/pix) of HR Lyr on Aug 26.93 UT with the Asiago 1.22m telescope. The spectrum is characterized by a smooth bluish continuum with superimposed strong emission lines. Hydrogen Balmer lines are visible in emission down to at least H13 (3734 Ang). Most prominent HeI emission lines are 7065, 6678, 5876, 5016 and 4922 Ang. The intensity of HeII 4686 is 0.32 times that of Hbeta. In units of 10(-15) erg cm-2 s-1, the integrated flux of Halpha, Hbeta, HeI 5876, HeII 4686 emission lines is 10.5, 6.0, 1.0, 1.8, respectively, and their E.W. 22.1, 8.4, 1.8, 2.5 Ang, respectively. The spectral appearance of HR Lyr during a dimming episode is therefore quite different from that exhibited during normal quiescence, when a featureless blue continuum dominates the optical spectrum with no trace of emission lines (Humason 1938, ApJ 88, 228; Kraft 1964, ApJ 139, 457; Williams 1983, ApJS 53, 523). A weak emission in Halpha has been reported for several quiescence epochs by Honeycutt et al (2014), who measured an average E.W.=6.6 Ang. During the current episode, the E.W. of Halpha emission has therefore more than tripled, as if the dimming affects the underlying blue continuum but not the emission lines.